MALDI- and ESI-MS of the HDL apolipoproteins; new isoforms of apoA-I, II

Apolipoproteins isolated from human high density lipoprotein were analyzed by matrix-assisted laser desorption (MALDI) and electrospray ionization mass spectrometry (ESI-MS). The long-term objective of this study is to identify and characterize new apolipoproteins and isoforms for clinical studies incorporating some of the latest advances in analytical chemistry methodology including mass spectrometry. The focus of this paper is on developing an understanding of the link between MALDI and ESI-MS data in the analysis of this complex mixture of proteins. Forty-nine peaks were observed in the MALDI spectrum and 11 species in the ESI-MS spectrum. The MALDI spectra consisted of peaks corresponding to known apoA and apoC molecular ion adducts and 11 species that could not be identified. The ESI-MS data provided excellent confirmation of MW values for the prominent ions in the spectrum. New isoforms of apoA-I and apoA-II were observed corresponding to truncation of C-terminal neutral amino acids. A new kind of isoform of apoA-II was discovered where the double strand is cleaved in vivo and the single strand is cysteinylated at the cleavage site. Confirmation was obtained by studying the change in the MALDI spectrum after performic acid oxidation. Charge-state distributions in the ESI-MS spectra were used to confirm the identity of this new isoform. The charge state distribution in ESI-MS was found to correlate with the number of basic residues in the apolipoprotein.